An IRS rule disallowing a loss if you rebuy the same security within 30 days.
The wash-sale rule blocks you from claiming a tax loss if you buy the same or a "substantially identical" security within 30 days before or after the sale. The disallowed loss is added to the cost basis of the replacement shares. It is the main constraint to respect when tax-loss harvesting.
Selling a fund for a loss and rebuying it a week later disallows the loss under the wash-sale rule.